Phylogenetic relationships of mid-oceanic ridge and continental lineages of lasaea spp. (Mollusca: Bivalvia) in the northeastern Atlantic

Direct-developing lineages of the genus Lasaea are common constituents of both oceanic island and continental rocky shore crevice faunas in the eastern North Atlantic. We utilized mitochondrial gene sequence variation to flesh out the phylogenetic relationships of individuals sampled from 2 Macaronesian archipelagos (Azores, Madeira) and from downstream continental (Iberian) sites. There was no evidence for colonization of the islands by upstream western North Atlantic congeners. Of 5 Lasaea clades detected in Iberia, 1 was also present on Madeira, whereas 4 of the 5 had representatives on the Azores. Madeira did not share haplotypes with the other sampling locations. In contrast, the Azorean and Iberian samples shared multiple haplotypes and our phylogenetic tree topologies were consistent with a minimum of 6 inferred migration events across the > 1400 km oceanic expanse separating these 2 regional populations. Three of the putative migration events involved a predominantly island clade whose topology Was consistent with colonization by ancestral continental lineages, extensive island cladogenesis, and secondary downstream migrations back to the mainland. The remaining 3 inferred migration events were distributed across the tips of the phylogenetic trees, a topology consistent with evolutionarily recent migrations against the prevailing current fields. Our results indicate that the pattern of easterly surface flow in the study area may generate differentially effective dispersal filters downstream of the Azorean and Madeiran archipelagos. Evidence for countercurrent migration in marine populations should be assessed in light of the totality of surface-flow patterns in the study system, not merely the prevailing one.